#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
using ll = long long;

int main(void){
  int n, m; cin >> n >> m;
  vector<string> s(n);
  vector<int> rem;
  for(int i=0; i<n; i++){
    cin >> s[i];
    rem.push_back(i);
  }
  string ans(m, 'G');
  auto f=[&](int a[3]){
    if(a[0]==0) return 'C';
    if(a[1]==0) return 'P';
    if(a[2]==0) return 'G';
    return '!';
  };
  for(int i=0; i<m; i++){
    vector<int> old;
    swap(old, rem);
    int cnt[3]={};
    for(auto p:old){
      if(s[p][i]=='G') cnt[0]++;
      else if(s[p][i]=='C') cnt[1]++;
      else cnt[2]++;
    }
    char x=f(cnt);
    if(x=='!'){
      cout << -1 << endl; return 0;
    }
    ans[i]=x;
    for(auto p:old)if(s[p][i]==x) rem.push_back(p);
    if(rem.empty()) break;
  }
  cout << ans << endl;
  return 0; 
}